BINGHAMTON, N.Y. — A gunman barricaded the back door of a community center with his car and then opened fire on a room full of immigrants taking a citizenship class, killing 13 people before apparently committing suicide, officials said.

Investigators said they had yet to establish a motive for Friday's massacre, which was at least the fifth deadly mass shooting in the U.S. in the past month alone.

The attack came just after 10 a.m. local time at the American Civic Association, which helps immigrants with citizenship, resettlement and family reunification in Binghamton, a city of about 47,000 situated 140 miles northwest of New York City.

Police Chief Joseph Zikuski said the gunman parked his car against the back door, "making sure nobody could escape," then stormed through the front, shooting two receptionists, apparently without a word.

The killer, believed to be a Vietnamese immigrant, then entered a room just off the reception area and fired on a citizenship class.

"The people were trying to better themselves, trying to become citizens," the police chief said.

One receptionist was killed, while the other, shot in the abdomen, pretended to be dead and then crawled under a desk and called police, he said.

The rest of those killed were shot in the classroom. Four people were critically wounded.

The man believed to have carried out the attack was found dead with a self-inflicted gunshot wound in an office, a satchel containing ammunition slung around his neck, authorities said. Police found two handguns — a 9mm and a .45-caliber — and a hunting knife.

Pakistani Taliban militant leader Baituallah Mehsud on Saturday claimed responsibility for the attack. However, he provided no evidence to support the claim.

Thirty-seven people in all made it out of the building, including 26 who hid in the boiler room in the basement, cowering there for three hours while police methodically searched the building and tried to determine whether the gunman was still alive and whether he was holding any hostages, Zikuski said.

Those in the basement stayed in contact with police by cell phone, switching from one phone to another when their batteries ran out, Zikuski said. Others hid in closets and under desks.

Police heard no gunfire after they arrived but waited for about an hour before entering the building to make sure it was safe for officers. They then spent two hours searching the building.

Most of the people brought out couldn't speak English, the chief said.

Alex Galkin, an immigrant from Uzbekistan, said he was taking English classes when he heard a shot and quickly went to the basement with about 20 other people.

"It was just panic," Galkin said.

Zhanar Tokhtabayeva, a 30-year-old from Kazakhstan, said she was in an English class when she heard a shot and her teacher screamed for everyone to go to the storage room.

"I heard the shots, every shot. I heard no screams, just silence, shooting," she said. "I heard shooting, very long time, and I was thinking, when will this stop? I was thinking that my life was finished."

Gov. David Paterson said the massacre was probably "the worst tragedy and senseless crime in the history of this city." Noting mass killings in Alabama and Oakland, California, last month, he said: "When are we going to be able to curb the kind of violence that is so fraught and so rapid that we can't even keep track of the incidents?"

Center officials issued a statement Friday night saying they were "stricken with grief about today's horrific assault and share this grief with the victims' families, our community and the entire nation."

'Senseless violence'
In Baden-Baden, Germany, President Barack Obama said he was shocked and saddened by the deadly mass shooting, calling it an "act of senseless violence."

The president, who is traveling in Europe, said he and his wife, Michelle, were praying for the victims, their families and the people of Binghamton.

The suspected gunman carried ID with the name of 42-year-old Jiverly Voong, of nearby Johnson City, New York, but that was believed to be an alias, said a law enforcement official, speaking on condition of anonymity.

A second law enforcement official, also speaking on condition of anonymity, said the two handguns were registered to Jiverly Wong, another name the man used. Both officials were not authorized to speak publicly.

Initial reports suggested Voong had recently been let go from IBM. But a person at IBM said there was no record of a Jiverly Voong ever working there.

The police chief would not confirm the name of the dead man with the ammunition satchel, saying authorities were still trying to establish with certainty that he was the gunman.

"We have no idea what the motive is," Zikuski said.

He said the suspected gunman "was no stranger" to the community center and may have gone there to take a class.

A woman who answered the phone at a listing for Henry D. Voong said she was Jiverly Voong's sister but would not give her name. She said her brother had been in the country for 28 years and had citizenship.

Friday evening, police searched Voong's house and carried out three computer hard drives, a brown canvas rifle case, a briefcase, a small suitcase and several paper bags.

Crime scene tape was stretched across the street about 20 yards from the house, and a steady rain fell as two state troopers stood guard to keep anyone but neighborhood residents from entering the dead-end street.

Waiting outside a Catholic Charities office where counselors were tending to relatives of victims, Omri Yigal said his wife, Delores, was taking English lessons when the gunman attacked. He had no word on what happened to her.

He finally left the center feeling sullen shortly before 8 p.m.

"They told me they don't have much hope for me," the Filipino immigrant said before going home to wait for a telephone call.

The American Civic Association helps immigrants in the Binghamton area with citizenship, resettlement and family reunification.

The Binghamton area was the home to Endicott-Johnson shoe company and the birthplace of IBM, which between them employed tens of thousands of workers before the shoe company closed a decade ago and IBM downsized in recent years.

A string of attacks in the U.S. in the last month left 44 people dead in all.

A gunman killed 10 people and himself in Samson, Alabama; shootings that began with a traffic stop in Oakland, California, left four police officers and the gunman dead; an apparent murder-suicide in Santa Clara, California, left six dead; and a gunman went on a rampage at a North Carolina nursing home Sunday, killing seven elderly residents and a nurse who cared for them.

The Associated Press, Reuters and NBC News' Mustaq Yusufzai contributed to this report.